Organic Reactions on Silica in Water

Metallosurfactant aggregates: Structures, properties, and potentials for multifarious applications

Metallosurfactants offer important scientific and technological advances due to their novel interfacial properties. As a special class of structures formed by the integration of metal ions into amphiphilic surfactant molecules, these metal-based amphiphilic molecules possess both organometallic and surface chemistries. This review critically examines the structural transitions of metallosurfactants from micelle to vesicle upon metal coordination. The properties of a metallosurfactant can be changed by tuning the coordination between the metal ions and surfactants. The self-assembled behavior of surfactants can be controlled by selecting transition-metal ions that enhance their catalytic efficiency in environmental applications by applying a hydrogen evolution reaction or oxygen evolution reaction. We present the different scattering techniques available to examine the properties of metallosurfactants (e.g., size, shape, structure, and aggregation behavior). The utility of metallosurfactants in catalysis, the synthesis of nanoparticles, and biomedical applications (involving diagnostics and therapeutics) is also explored.

Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface-selective vibrational (χ(2)) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the χ(2) (Imχ(2)) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt-concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pKa value (4.8 ± 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Imχ(2) spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

Rhodium-catalyzed selenylation and sulfenylation of quinoxalinones 'on water'

A rhodium-catalysed, regioselective synthetic methodology for selenylation and sulfenylation of 3-phenyl quinoxolinones has been developed through N-directed C-H activation in the presence of silver triflimide, and silver carbonate using dichalcogenides 'on water'. The methodology has been proven to be efficient, regioselective and green. Using this method, a range of selenylations and sulfenylations of the substrates has been carried out in good to excellent yields. Further, late-stage functionalisation produced potential anti-tumour, anti-fungal and anti-bacterial agents making these compounds potential drug candidates.

Pickering Emulsion Catalysis: Interfacial Chemistry, Catalyst Design, Challenges, and Perspectives

Pickering emulsions are particle-stabilized surfactant-free dispersions composed of two immiscible liquid phases, and emerge as attractive catalysis platform to surpass traditional technique barrier in some cases. In this review, we have comprehensively summarized the development and the catalysis applications of Pickering emulsions since the pioneering work in 2010. The explicit mechanism for Pickering emulsions will be initially discussed and clarified. Then, summarization is given to the design strategy of amphiphilic emulsion catalysts in two categories of intrinsic and extrinsic amphiphilicity. The progress of the unconventional catalytic reactions in Pickering emulsion is further described, especially for the polarity/solubility difference-driven phase segregation, "smart" emulsion reaction system, continuous flow catalysis, and Pickering interfacial biocatalysis. Challenges and future trends for the development of Pickering emulsion catalysis are finally outlined.

Stabilization of ruthenium on biochar-nickel magnetic nanoparticles as a heterogeneous, practical, selective, and reusable nanocatalyst for the Suzuki C-C coupling reaction in water

Waste recycling and the use of recyclable and available catalysts are important principles in green chemistry in science and industrial research. Therefore in this study, biochar nanoparticles were prepared from biomass pyrolysis. Then, they were magnetized with nickel nanoparticles to improve their recycling. Further, the magnetic biochar nanoparticles (biochar-Ni MNPs) were modified by dithizone ligand and then applied for the fabrication of a ruthenium catalyst (Ru-dithizone@biochar-Ni MNPs). This nanocatalyst was characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), wavelength dispersive X-ray spectroscopy (WDX), N₂ adsorption–desorption isotherms, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM) techniques. The XRD studies of Ru in the nanocatalyst showed that the crystalline structure of ruthenium in the Ru-dithizone@biochar-Ni MNPs was hcp. Another principle of green chemistry is the use of safe and inexpensive solvents, the most suitable of which is water. Therefore, the catalytic activity of this catalyst was investigated as a practical, selective, and recyclable nanocatalyst in the Suzuki carbon–carbon coupling reaction in aqueous media. The VSM curve of this catalyst showed that it could be easily recovered using an external magnet, and recycled multiple times. Also, VSM analysis of the recovered catalyst indicated the good magnetic stability of this catalyst after repeated use.

Waste recycling and the use of recyclable and available catalysts are important principles in green chemistry in science and industrial research.

Sustainable C–H activation approach for palladium-catalyzed, regioselective functionalization of 1-methyl-3-phenyl quinoxaline-2(1H)-ones in water

An environment-friendly approach for regioselective acylation of 1-methyl-3-phenyl quinoxaline-2(1H)-ones was developed using water as a solvent. The protocol exhibits a wide substrate scope and employs commercially available, non-toxic acyl surrogates.

Influence of water content on the [2σ+2σ+2π] cycloaddition of dimethyl azodicarboxylate with quadricyclane in mixed methanol-water solvents from QM/MM Monte Carlo simulations

Mixed quantum mechanics/molecular mechanics Monte Carlo (QM/MM/MC) simulations combined with the free energy perturbation (FEP) theory have been performed to investigate the mechanism and solvent effect of the [2σ+2σ+2π] cycloaddition reaction between dimethyl azodicarboxylate and quadricyclanes in the binary mixture solvents of methanol and water by varying the water content from 0 to 100 vol%. The two-dimensional potentials of mean force (2D PMF) calculations demonstrated that the mechanism of the reaction is a collaborative asynchronous procedure. The transition structures do not show large variation among different solvents. The calculated free energies of activation indicated that the QM/MM/MC method reproduced well the tendency of rate enhancement from pure methanol to methanol-water mixtures to "on water" with the water content increasing obtained in the experimental observation. The analyses of the energy pair distribution and radial distribution functions illustrated that hydrogen bonding plays an indispensable role in the stabilization of the transition structures. According to the results in methanol-water mixtures at different volume ratios, it is clear that the site-specific hydrogen bond effects are the central reason which leads to fast rate increases in progressing from a methanol-water volume ratio of 3 : 1 to 1 : 1. This work provides a new insight into the solvent effect for the [2σ+2σ+2π] cycloaddition reaction.

Integrated Cobaloxime and Mesoporous Silica-Supported Ruthenium/Diamine Co-Catalysis for One-Pot Hydration/Reduction Enantioselective Sequential Reaction of Alkynes

This study developed a cost-efficient hydration/asymmetric transfer hydrogenation (ATH) process for the one-pot synthesis of valuable chiral alcohols from alkynes. During this process, the initial homogeneous cobaloxime-catalyzed hydration of alkynes was followed by heterogeneous Ru/diamine-catalyzed ATH transformation of the in-situ generated ketones, which provided varieties of chiral alcohols in good yields with up to 99% ee values. The immobilized Ru/diamine catalyst could be recycled at least three times before its deactivation in the sequential reaction system. This work shows a general method for developing one-pot asymmetric sequential catalysis towards sustainable organic synthesis.

Ultrafast Dynamics and Liquid Structure in Mesoporous Silica: Propagation of Surface Effects in a Polar Aprotic Solvent

Enhancement of processes ranging from gas sorption to ion conduction in a liquid can be substantial upon nanoconfinement. Here, the dynamics of a polar aprotic solvent, 1-methylimidazole (MeIm), in mesoporous silica (2.8, 5.4, and 8.3 nm pore diameters) were examined using femtosecond infrared vibrational spectroscopy and molecular dynamics simulations of a dilute probe, the selenocyanate (SeCN^-) anion. The long vibrational lifetime and sensitivity of the CN stretch enabled a comprehensive investigation of the relatively slow time scales and subnanometer distance dependences of the confined dynamics. Because MeIm does not readily donate hydrogen bonds, its interactions in the hydrophilic silanol pores differ more from the bulk than those of water confined in the same mesopores, resulting in greater structural order and more dramatic slowing of dynamics. The extent of surface effects was quantified by modified two-state models used to fit three spatially averaged experimental observables: vibrational lifetime, orientational relaxation, and spectral diffusion. The length scales and the models (smoothed step, exponential decay, and simple step) describing the transitions between the distinctive shell behavior at the surface and the bulk-like behavior at the pore interior were compared to those of water. The highly nonuniform distributions of the SeCN^- probe and antiparallel layering of MeIm revealed by the simulations guided the interpretation of the results and development of the analytical models. The results illustrate the importance of electrostatic effects and H-bonding interactions in the behavior of confined liquids.

Dinuclear Copper Complex Immobilized on a Janus-Type Material as an Interfacial Heterogeneous Catalyst for Green Synthesis

We herein describe a rational design of a heterogeneous catalyst composed of a dinuclear cuprate anion being immobilized electrostatically on one surface of Janus-type nanosheets while the other surface is decorated with highly hydrophobic octyl groups. The catalyst was found to be well dispersible in the organic phase of a biphasic aqueous/organic mixture. It was characterized by means of elemental analysis, atomic absorption spectroscopy, mass spectrometry, N₂ absorption-desorption analysis, thermogravimetric analysis, scanning electron microscopy (SEM), and solid-state ¹³C and ²⁹Si cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy. The Janus nature of the catalyst was investigated by employing a selective surface labeling method and by means of SEM. The catalyst shows higher activity compared to a non-Janus analogue in a biphasic synthesis. It was successfully used for the azide-alkyne cycloaddition and the Chan-Lam C-N coupling reaction. In addition, new and simple ways have been established for the production of a coumarin-triazole derivative and for the synthesis of the biologically active compound Monastrol via a solvent-free Biginelli reaction. The role of the dinuclear copper centers is discussed mechanistically.

Ultrasmall amphiphilic zeolitic nanoreactors for the aerobic oxidation of alcohols in water

Organic reactors in a green solvent (water) is the goal of sustainable development. Green nanoreactors with excellent amphiphilicity and catalytic activity are strongly desired. Herein, a novel amphiphilic nanoreactor Pd@amZSM-5 with ultrasmall size has been successfully synthesized via a simple one-step oil bath method, subjected to the modification-etching-modification strategy and in situ reduction of Pd2+. Ultrasmall Pd@amZSM-5 nanoreactors (60 nm) with hierarchical structures showed outstanding amphiphilicity for forming Pickering emulsions with fine uniform droplets (50 μm). Fine droplets formed short diffusion distances, which can significantly improve the catalytic activity in biphasic reactions. Moroever, the ultrasmall Pd@amZSM-5 nanoreactors demonstrated excellent catalytic activity for the selective oxidation of alcohols in water using air as the oxidant. Alkali was not present in the reaction system. The hydrophilic aminopropyl groups on the surface of the Pd@amZSM-5 nanoreactors not only changed the affinity of the zeolite surface and provided targeting points for Pd nanoparticles but also provided an alkaline environment for the selective oxidation of alcohols. The ultrasmall Pd@amZSM-5 nanoreactors presented excellent universality for aromatic alcohols (with >90% conversion and >90% selectivity) and allylic alcohols (with 100% conversion and 100% selectivity).

Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies

Hydrophobicity/hydrophilicity of aqueous interfaces at the molecular level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via density functional theory-molecular dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the number of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of molecular hydrophobicity at aqueous interfaces.

Growth and Atomic-Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

The present review reports on the preparation and atomic-scale characterization of the thinnest possible films of the glass-forming materials silica and germania. To this end state-of-the-art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side-by-side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.

Si-thiol supported atomic-scale palladium as efficient and recyclable catalyst for Suzuki coupling reaction

Atomic-scale catalysts leverage the advantages of both heterogeneous catalysts for their stability and reusability and homogeneous catalysts for their isolated active sites. Here, a palladium catalyst supported by Si-thiol, a commercially available mercaptopropyl-modified and TMS-passivated amorphous silica, was synthesized and characterized by SEM,TEM, aberration-corrected STEM-HAADF, XRD, FT-IR and XPS. Statistical analysis revealed that the catalytic Pd species predominantly consisted of intermediate sized nanoparticles (<2 nm), small amounts of essentially isolated atoms (ca. 0.1 nm), and limited amounts of somewhat larger nanoparticles (<5 nm). The nanoscale atomic clusters dominated the reactivity and served as the key active sites for Suzuki coupling. The outcomes of the reaction were greatly affected by the choice of solvents, and Pd/Si-thiol was demonstrated to be reusable for more than three times without a noticeable loss of catalytic activity. [Formula: see text].

Nitroepoxide ring opening with thionucleophiles in water: synthesis of α-xanthyl ketones, β-keto sulfones and β-keto sulfonic acids

Nitroepoxide ring opening with thionucleophiles such as potassium xanthates, sodium aryl sulfinates and sodium bisulfite in water is investigated to provide the corresponding α-xanthyl-α-aryl-2-propanones, β-keto sulfones and β-keto sulfonic acids.

Catalysis with magnetically retrievable and recyclable nanoparticles layered with Pd(0) for C–C/C–O coupling in water

Nanoparticles layered with palladium(0) were prepared from nano-sized magnetic Fe₃O₄ by coating it with silica and then reacting sequentially with phenylselenyl chloride under an N₂ atmosphere and palladium(ii) chloride in water. The resulting Fe₃O₄@SiO₂@SePh@Pd(0) NPs are magnetically retrievable and the first example of NPs in which the outermost layer of Pd(0) is mainly held by selenium. The weight percentage of Pd in the NPs was found to be 1.96 by ICP-AES. The NPs were authenticated via TEM, SEM-EDX, XPS, and powder XRD and found to be efficient as catalysts for the C–O and C–C (Suzuki–Miyaura) coupling reactions of ArBr/Cl in water. The oxidation state of Pd in the NPs having size distribution from ∼12 to 18 nm was inferred as zero by XPS. They can be recycled more than seven times. The main features of the proposed protocols are their mild reaction conditions, simplicity, and efficiency as the catalyst can be separated easily from the reaction mixture by an external magnet and reused for a new reaction cycle. The optimum loading (in mol% of Pd) was found to be 0.1–1.0 and 0.01–1.0 for O-arylation and Suzuki–Miyaura coupling, respectively. For ArCl, the required amount of NPs was more as compared to that needed for ArBr. The nature of catalysis is largely heterogeneous.

Fe₃O₄@SiO₂@SePh@Pd(0) (Pd, 1.96%) as the first example of NPs having a Pd(0) layer held by selenium can execute C–C/C–O coupling in 2–6 h (80 °C).

Dynamics in hydrophilic and hydrophobic molecular chains tethered to MCM41-type mesoporous silica upon wetting and dehydration processes

Surface modified mesoporous silica materials are important materials for heterogeneous catalysis and are attracting attention as potential drug carriers. The functionality of these materials relies on the physical and chemical properties of the tethers attached to MCM41 silica surface. These chemically linked tails act as molecular brushes, that can capture pollutant molecules, anchor points for catalysts and can host drug molecules. To utilize the full potential of the tailored silica surfaces, one should infer their properties at different levels of solvation. Here, ¹H MAS NMR spectroscopy is used to monitor the dynamic properties of two modified MCM41 materials, an aminopropyl tethered MCM41 and an octyl tethered MCM41, through the process of controlled hydration. The surface site resolved measurements demonstrate how the chemical nature of the two tethers governs the way water molecules are directed to the different sites in the porous materials.

Effects of solvents on the DACBO-catalyzed vinylogous Henry reaction of isatin with 3,5-dimethyl-4-nitroisoxazole "on-water" and in solution from QM/MM MC simulations

The mechanism of the DABCO-catalyzed vinylogous Henry reaction of isatin with 3,5-dimethyl-4-nitroisoxazole and solvent effects on it have been investigated using density functional theory (DFT) methods and QM/MM Monte Carlo (MC) simulation under “on-water” conditions as well as in methanol and THF solutions. The DFT calculations concluded that Path A, in which DABCO directly catalyzes the reaction of isatin 1a with 3,5-dimethyl-4-nitroisoxazole 2 in water, is the most favorable and the first step, the proton transfer process, is the rate-determining step for the reaction. For the roles of solvents in the reaction, QM/MM MC simulations using free energy perturbation theory and PDDG/PM3 as the QM method have been utilized to predict the free energy profiles. The results indicated that the QM/MM method reproduced well the large rate increases on-water. Solute–solvent energy pair distribution and radial distribution functions were also analyzed and illustrated that hydrogen bonding plays a significant role in stabilizing the transition structures. This work reveals the feasible reaction mechanisms and provides new insight into solvent effects for the DACBO-catalyzed vinylogous Henry reaction.

QM/MM MC simulations indicate that the special hydrogen bonding effects are the main source of the rate enhancement of the title reaction on water.

Robust Bijels for Reactive Separation via Silica-Reinforced Nanoparticle Layers

Pickering emulsions have been successfully used as media for catalysis and separation. However, simultaneous reaction and separation cannot be performed in a continuous mode in these systems, because reagents cannot be readily loaded into or recovered from the dispersed phase. Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are continuous throughout the structure, have potential as media for simultaneous reaction and separation in a continuous mode. In this work, we take a major step toward realizing this vision by demonstrating the ability of bijels to be used in reactive separation performed in a batch fashion. To perform effectively, bijels must maintain their morphology and interfacial mass transfer properties during reaction. To strengthen the bijels, we modify the solvent transfer-induced phase separation (STRIPS) method to make bijels resistant to mechanical stresses and prevent detachment of nanoparticles from the oil/water interface due to pH changes by chemically fusing the interfacial nanoparticles. The reinforced bijel is successfully tested in base-catalyzed hydrolysis of esters and remains robust under these challenging conditions.

Molecular hydrophobicity at a macroscopically hydrophilic surface

Interfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atom toward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle.

A carboxylic acid functionalized SBA-15 supported Pd nanocatalyst: an efficient catalyst for hydrogenation of nitrobenzene to aniline in water

The catalytic performance of a PdNPs/SBA-COOH has been investigated for the first time in the selective hydrogenation of nitrobenzene with hydrous hydrazine at RT in water medium.

Thermodynamic and molecular origin of interfacial rate enhancements and endo-selectivities of a Diels-Alder reaction

Organic reactions in general display large rate accelerations when performed under interfacial conditions, such as on water or at ionic liquid interfaces. However, a clear picture of the physicochemical factors responsible for this large rate enhancements is not available. To gain an understanding of the thermodynamic and molecular origin of these large rate enhancements, we performed a Diels-Alder reaction between cyclopentadiene and methyl acrylate at ionic liquid/n-hexane interfaces. This study describes, for the first time, a methodology for the calculation of the activation parameters of an interfacial reaction. It has been seen that the energy of activation for an interfacial reaction is much smaller than that of the corresponding homogeneous reaction, resulting into the large rate acceleration for the interfacial reaction. Furthermore, the study describes the effects of the alkyl chain length of ionic liquid cations, the extent of heterogeneity, and the polarity of ionic liquids on the rate constants and stereoselectivity of the reaction.

A Ni(iii) complex stabilized by silica nanoparticles as an efficient nanoheterogeneous catalyst for oxidative C-H fluoroalkylation

We have developed Ni(III)-doped silica nanoparticles ([(bpy)xNi(III)]@SiO2) as a recyclable, low-leaching, and efficient oxidative functionalization nanocatalyst for aromatic C-H bonds. The catalyst is obtained by doping the complex [(bpy)3Ni(II)] on silica nanoparticles along with its subsequent electrooxidation to [(bpy)xNi(III)] without an additional oxidant. The coupling reaction of arenes with perfluoroheptanoic acid occurs with 100% conversion of reactants in a single step at room temperature under nanoheterogeneous conditions. The catalyst content is only 1% with respect to the substrates under electrochemical regeneration conditions. The catalyst can be easily separated from the reaction mixture and reused a minimum of five times. The results emphasize immobilization on the silica support and the electrochemical regeneration of Ni(III) complexes as a facile route for developing an efficient nanocatalyst for oxidative functionalization.